DESCRIPTION (applicant's abstract): The ventromedial nucleus of the hypothalamus (VMH) is a sexually dimorphic region that controls sexual behavior in female rats, typified by the lordosis reflex. Females are sexually receptive only when estrogen is present, and estrogen receptors in the ventrolateral subdivision of the VMH (vlVMH) mediate the effects of estrogen on sexual behavior. Male rats, in contrast, do not exhibit the lordosis response, even when castrated and treated with estrogen. The general goal of this proposal is to investigate the local circuitry in the vlVMH that couples estrogen action with the pathways that execute the lordosis reflex. The specific aims are: 1) To test the hypothesis that the lordosis-relevant efferents are segregated from, and are innervated by, the estrogen receptor-containing neurons; 2) To test the hypothesis that estrogen induces dendritic spines in lordosis-relevant efferent neurons, but not estrogen receptor-containing neurons, in the vlVMH; and 3) To test the hypothesis that sex differences in specific components of the VMH local circuitry mediate the sex-dependent effect of estrogen on spine density in the VMH. The methods will include quantification and localization of neurons visualized by transneuronal tracing, conventional anterograde and retrograde tracing, confocal microscopic analysis of dendritic spine density on Lucifer yellow-filled cells, and the combination of these techniques with immunocytochemistry. The results of these studies may illuminate the effects of estrogen on neural function.